Science —

The fastest hologram in the world

One of the hot areas in laser physics at the moment is really a fusion of two research streams: plasma physics, and high intensity laser physics. The reason for this fusion is two fold; firstly, a plasma is generated when these lasers strike matter and, secondly, the resulting laser-plasma interaction has many potential applications. To give an example; these plasmas can be used to generate coherent, extreme UV and X-Ray light—an exciting prospect if you happen to be in the business of obeying Moore's Law at all costs. Another example is electron and proton acceleration, which is currently the domain of cyclotrons and huge linear accelerators. Laser driven accelerators achieve huge accelerations (up to 1GeV) in just a few millimeters of plasma.

Unfortunately, one of the things holding the field back is the lack of data on the formation of the plasma. The laser pulse is typically just a few femtoseconds (10-15s) long and the plasma is created during and/or shortly after the laser pulse. No camera in the world can take pictures fast enough to observe the formation and dynamics of the plasma.

One way to get around this problem is to use the laser pulse itself to probe the plasma. Essentially, a small portion of the laser pulse is split off and delayed with respect to the main pulse. Changes to the medium are then inferred by the behavior of the weak probe pulse. By changing the delay, the medium and plasma are sampled at different times with respect to the main pulse. However, these setups are incredibly complicated—an accuracy of 1fs implies that the path difference between the main pulse and probe pulse has to be be accurate to within 300nm. Furthermore, the full measurement takes a long time since the delay time must be adjusted between each shot and the material targeted for destruction changed. Which brings replication to the fore: it is always difficult to be sure that the main pulse, probe pulse, material, and focusing conditions are identical for each measurement.

A large multinational team of researchers has just published a new method for making these measurements in Nature. They make use of an observation originally performed by Newton and explained by Young (of double slit fame). Newton's observation was that a spot of light reflected from a mirror (with the reflective coating on the back surface) exhibited rings around the reflected spot. These rings are due to the interference of light scattered from dust on the front surface of the mirror. The researchers constructed the equivalent of a dusty mirror for their soft X-ray laser. The dust is replaced by polystyrene balls, onto which the laser is focused. Some light is scattered directly from the surfaces of the balls, while the rest passes through—on the way through it wreaks havoc and turns the balls into a plasma—to be reflected from a mirror. The light passes back through the balls and the combination of that light with the light originally scattered, creates an interference pattern. The formation of plasma and other factors can then be determined by examining the interference pattern. To obtain variable delays from a single measurement, the membrane holding the balls is placed at an angle to the mirror so that the time delay varies on a single axis of the interference pattern. The detected image is really a hologram that encodes the changes of the polystyrene balls as a function of time delay in a single flash from the laser.

As I said, this is one of the hottest areas in laser physics right now, so I expect to see this technique replicated on more interesting systems in the near future. I will be waiting with antici...pation to learn new things about the formation of plasmas from this research.

Nature, 2007, doi:10.1038/nature06049

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Chris Lee
Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He is delocalised, living and working in Eindhoven and Enschede, the Netherlands. Emailchris.lee@arstechnica.com//Twitter@exMamaku